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Monday, July 2, 2012

On its own, a Higgs discovery would be grim

Why the Higgs is cool

If rumours are to be believed, then, in two days time, CERN will announce the discovery of a new particle and it will be called Higgs. To the degree that the discovery of any new particle is a pretty big deal, this will be a pretty big deal.

To put things into perspective, not only will this be the discovery of an entirely new particle, if the standard model of particle physics is correct, this will also be the discovery of an entirely new fundamental particle. That is, it won't be made up of any constituent pieces. Also, the field that it will be excited from will not have been directly detected ever before. And that's not even it. Other aspects of the Higgs are also completely new. For example, the way it behaves when you rotate it will be unique amongst all the fundamental particles we've discovered so far, which is quite curious because its rotational properties will be the simplest (i.e. it has no spin at all).

So, irrespective of everything I'm about to write I want to first stress the following: the discovery of a Higgs-like particle is pretty damn cool and a great achievement of exploration for humanity.

Beyond the hype

However, the Higgs is no God particle and it is not the origin of all the mass in the universe (or even a significant proportion of it). No great mysteries of the universe are about to be solved on Wednesday. The Higg's significance in our understanding of the universe is similar to the understanding gained when the last piece of a jigsaw is finally placed in a puzzle. Placing that last piece produces an enormous amount of cathartic pleasure (more so than any other individual piece). But, the image in the puzzle has become clear long before that final piece is placed. The role the Higgs plays in the standard model of particle physics is to break a certain symmetry in nature, the electroweak symmetry. All the other pieces of this broken symmetry have been found, some quite a long time ago.

The reason the Higgs has been elevated to this high level of importance in public discussions is that it was the last piece of this well understood puzzle to be discovered. The electroweak symmetry and the fact that it gets broken are themselves very interesting things. In fact, they are deeply fascinating things. The problem is that they were understood as early as the 1960s. The Nobel prize was awarded for their description in 1979, thirty three years ago. The correct perspective with which to view this discovery is not as the beginning of a new era of understanding, but the end of an earlier one (no matter what is heard at the water-cooler of the internet).

This is not to say that the Large Hadron Collider (LHC) won't make other discoveries that would herald such a new era. If, for example, the LHC were to create and detect dark matter, or supersymmetry, this truly would be revolutionary. It would be the first evidence of a particular new fundamental interaction. In fact, any discovery of anything except the Higgs at the LHC would be revolutionary.

However, if all the LHC finds is the Higgs then this would be a tragedy for our generation's pursuit of knowledge. This tragedy would not be the LHC's fault, nor, in fact, anyone's fault. It would just be a cruel trick of nature. But, unfortunately, it would be a cruel trick that is our generation's to bear and would be devastating for our attempts to learn more about the fundamental aspects of the universe.

Why did we build the LHC?

A sensible reader might wonder to themselves why we even built the LHC then. That is, if discovering the Higgs would be cool, but wouldn't really further our understanding of nature at all, why build a collider just to find it? The answer is that the standard jigsaw puzzle of particle physics (the standard model), for which the Higgs is the last remaining piece is littered with things that seem unsatisfactory... and we wanted answers.

Many of the fundamental particles we've observed come in sets of three. The elements of each set are absolutely identical in every way except their mass. There is the electron, but there is also the muon and tau. There is the electron neutrino, but there is also the muon neutrino and tau neutrino. There is the down quark, but there is also the strange quark and bottom quark. There is the up quark, but there is also the charm quark and top quark. There is absolutely no known reason why these copies need to exist, they're just there. When the muon, the very first one of these surplus particles, was discovered in 1936, I. I. Rabi, a prominent physicist of the time (and future Nobel laureate) made the witticism “Who ordered that?” We still haven't answered Rabi's question.

80% of the matter in the universe consists of dark matter. We do not know what this dark matter is made of. Although we do know that it is not made of anything in the standard model. It is called dark matter because it doesn't interact with light; it was hoped that it would interact with the weak force. If so, it could be seen at the LHC. But there is no guarantee that it will interact with the weak force and even if it does, this is no guarantee that it will be discovered by the LHC. The energy at which dark matter interacts might simply be beyond the LHC's reach.

The universe is full of matter. Yet the way in which matter and anti-matter interact with other particles is almost perfectly symmetric. These two facts do not seem to be consistent. There must be some physics we haven't yet detected that can produce a universe consisting of only matter. In many theories the temperature of the universe when this asymmetry was generated would correspond to energies accessible by the LHC, but there is no guarantee that these theories are correct.

Neutrinos have mass. This we now know. What we don't know is what their masses are and how they got them. It is difficult to measure these masses directly because they are extremely small, and neutrinos interact incredibly weakly. Even if the neutrinos get mass the same way that other fundamental particles do (i.e. through the Higgs field), detecting the Higgs particle will not help elucidate what those masses are. If the Higgs mechanism is responsible, this is also somewhat startling. The mass a particle gets through this mechanism depends directly on how strongly it interacts with the Higgs field. The top quark has a mass of 173 GeV (G means \(10^9\)). The heaviest the neutrinos can be is 0.3 eV. That is \(6\times10^{11}\) times smaller. It would be great to know the origin of this hierarchy.

Finally, there is the thing known as the hierarchy problem. The mass of the Higgs should be much larger than it appears to be. If there is any new physics at higher energy scales (and the stuff I've just written should convince you that there is) then the mass of the Higgs should receive contributions from this new physics. We've known for decades that the standard model Higgs would need to have a mass comparable to the one that is now, perhaps, being measured at LHC energy scales. The hierarchy in the hierarchy problem is this hierarchy of energy scales. Why is the energy scale for the Higgs mass so small compared to any other, undiscovered, fundamental interactions? Note that, it is only the Higgs that is unprotected from these mass contributions at higher energy scales.

The hierarchy problem, more than anything else, has contributed to expectations that something else was waiting for us at LHC-like energy scales. The most popular new physics physicists hoped would be discovered at the LHC is a new fundamental symmetry of nature that would protect the Higgs mass from higher energy contributions (as well as doing quite a few other pretty cool things). This symmetry is called supersymmetry. However, for supersymmetry to fully solve the hierarchy problem it would need to have shown up at lower energy scales. If it shows up now, there will still be a little hierarchy problem.

Where does this leave us?

All of these problems and issues tell us that something else exists beyond the standard model, but there is simply no guarantee that this something occurs at energy scales we can probe with colliders here on Earth. The hierarchy problem strongly suggested there should have been new physics at (or, more precisely, below!) the energy scales probed by the LHC, but there have been no signs of supersymmetry yet.

And, without the hierarchy problem, there is simply no good reason that any of these new bits of fundamental physics should show up at energy scales ever examinable by those of us living today. They could occur at 10 times, or 100 times, or 1000 times the energy of the LHC, or more!

This doesn't mean that all the grand ideas we've come up with to explain these problems would be wrong, only that we, our generation, cannot test them. This also doesn't mean that they are somehow not science or “not even wrong”. I'm not sure there is even a Platonic thing called science, but, irrespective of that, these models are testable, eventually. One day, in the long distant future, these questions will definitely have been answered. The sad possibility is that by then the LHC would belong to a previous age, like Newton does to us now.

The poignant end to a method of exploration?

If this is true, that is, if the LHC finds the Higgs and the Higgs alone, it will be very difficult to convince anyone to build a bigger, larger energy scale, collider. Such a collider would cost billions and is not guaranteed of seeing anything new. There would probably (hopefully?) be one last roll of the dice with the International Linear Collider (ILC), which would collide electrons and positrons at LHC-like energies. It is much harder to accelerate a beam of positrons and electrons to the same energies as the protons in the LHC, but their eventual collisions are much cleaner. Therefore, the ILC could measure the properties of the Higgs field much more precisely than the LHC and for that alone it would be worthwhile building it. But after the ILC, if it too did not find any deviations from the standard model, that would be it, for generations.

To me, that is a tragically poignant possibility. I am reminded of the great explorers who sailed the Earth in the 15th to 17th centuries. For those 300 years the world was full of discovery and around every corner was a new continent, or a new island, or new sea to be explored. For 300 years, expertise was passed on from generation to generation and the sophistication of naval craft became greater and greater. But, one day, the Earth's oceans ran out of new things to discover and the age of exploration was over. All the great stories of ocean-faring discovery and exploration would still be there, but there would be no new ones written, ever.